Microphone transmitter



Jan. 8 11935.. H F, OLSON 1,987,433

MICROPHONE TRANSMITTER Filed July 13, 1931 2 Sheets-Sheet 1 INVENTOR HARRYF. OLSON A TORNEY Jan, 8,, W35. H. F. OLSON 11,987,413

MICROPHONE TRANSMITTER Filed July 13, 1931 2 Sheets-Sheet 2 one 0 0 o o 000 one o 0 0 o 00c :00 0 o 0 o o 000 loo 0 o o o 0 can I90 0 0 O o 0 can van 0 o o o 0 00a ace 0 0 o o 0 000 nab o o o o o 000 INVENTOR HARRYEOLSON ATTORNEY yatented 9 3935 UNITED STATES MICROPHONE TRANSMITTER Harry F. Olson, New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application 11117 13, 1931, Serial No. 550,373

5 Claims. (Cl- 179-178) This invention relates to a microphone transmitter especially adapted for use in talking moving picture work or other field where faithful reproduction or recording of sound is desired. In its preferred form it comprises a unit consisting of a housing on which a microphone and audio amplifier is mounted.

An object of the invention is to provide a microphone, preferably of the ribbon type, which is so arranged as to produce an electric current or potential whose magnitude varies proportionally to the pressure of the sound wave actuating it.

A further object is to provide a microphone which is directional and means for readily changing its position with respect to the source of sound. Such means is preferably operable from a point remote from the microphone, such as the location of the operator of the moving picture camera or the monitor of the recording.

A still further object is to provide an eflicient housing to protect the delicate component parts of the unit from being touched or injured by foreign objects. Preferably the housing portion protecting the microphone is a combination metal and fabric screen, which serves to shield it from electrostatic and electro-magnetic fields.

These, together with further objects of the invention will become apparent as the description thereof proceeds; Preferred embodiments of the invention are shown in the accompanying drawings, in which,

Figure 1 is a front view of an approved form of the apparatus embodying the invention;

Figure 2 is a front view of the apparatus shown in Figure 1 with the casings removed fromthe amplifier and microphone units;

Figure 3 is a view of the apparatus of Figure 1 arranged for control from a remote point;

Figure 4 is a sectional view showing the construction of the screens and their arrangement about the microphone;

Figure 5 is a sectional view of a modified form of screen arrangement for the microphone;

Figure 6 is a front view of a screen arrangement especially adapted for use with the device shown in Figure l; and,

Figure 7 is a cross sectional view of the screen shown in Figure 6 taken on the line M of Figure s.

As shown in Figures 1 and 2 the device comprises a main framework 1 on which are mounted the component parts of a three stage audio amplifier. Reference numeral 2 designates the input transformer of said amplifier; numeral 3 the capacitor pack thereof; numeral 4 the coupling condensers; and, numeral 5 the vacuum tubes which in the present instance are three UX-864 Radiotrons. The five prong plug 6 at the top of the framework serves to convey the filament current and plate current to the tubes and two of its terminals serve as output terminals for the amplifier signal leads. The apparatus within the upper part of framework 1 is covered and protected by a cylindrical metal covl0 of frame 10 so that it may be moved about a horizontal axis and clamped in any desired position by the clamping nut 14. The microphone transmitter 12 is preferably of the ribbon type mounted on screws 13 carried in the lower part 20 as disclosed and claimed in an application filed 2 in the name of Harry Olson, Serial No. 526,598. filed March 31, 1931. The ribbon 15 is shown secured at its top and bottom portions for vibration between the pole pieces 16, of electro-magnet 17 having the field coil 18-. To protect the transmitter from mechanical injury and to shield it from undesired electro-magnetic and electrostatic influences it may be enclosed in a casing or shield 19.

This casing is preferably made of sheet metal, such as, brass or aluminum of sufficient thickness to be strong enough to serve as a good mechanical protection for the transmitter. As shown in Figure 1, the lower portion of shield 19 has a plurality of apertures 20, these being provided to admit the sound wave to the ribbon. In one embodiment of the invention, thejoint area of these perforations was about A the total area of the shield. In this case the holes 20 had a diameter of about 5/64 inch and were spaced about y inch apart. The holes in the screen are preferably made of small size so that when the transmitter is used in a location where air or wind currents are present such currents will not enter the easing and cause motion of the ribbon 15, which motion would produce undesired audio currents in the output of the amplifier. The screen also serves to prevent foreign bodies which might be floating in the air from contacting with the ribbon and injuring it. The total area of the apertured portion of the screen is such that it does not prevent any appreciable attenuation of the highest audio frequencies desired to be recorded. I have found with the screen made as shown that frequencies as high as 10,000'cycles per second will operate a microphone and be amplified by the amplifier shown without any substantial attenuation.

I have found it preferable to provide perforations on the ends of the case as well as on its sides, as shown in Figure 1, since when the end perforations are omitted the end portions of the case tend to form aresonant chamber which woulddiscriminate strongly against certain frequencies and thereby cause distortion in the recording of the sound waves. 7

A preferred casing or shield to be used in connection with the shield shown in Figure 1 is shown in section in Figure 4. Here the outer rectangular metal casing 19 having apertures 20 has its entire side and bottom portions covered with a sheet of closely woven fabric 21 preferably made of silk. To hold the silk firmly. in contact with casing 19 so that it cannot move an interior framework 22 made preferably of wire gauze is provided. This framework is of such shape and size that when fitted within a screen 19 it holds the silk taut and prevents it from moving when air currents strike against it. I prefer to make the lining of a silk material, the threads of which are very fine and which have been very closely woven together so that only minute air spaces exist between the silk threads. when such a composite screen is about the microphone, the outer casing protects it against mechanical injury, and a silk lining assists its action in preventing air or wind currents from affecting the microphone. I also find that the silk serves admirably to prevent fine dust and iron particles which are usually present in the atmosphere from settling on the ribbon and in the air gap of the magnet and thereby interfering with its proper operation. I have found that a composite screen of the type described does not ofler any appreciable attenuation to audio frequencies of about 10,000 cycles per second. I have found it very desirable to use some means to prevent the silk lining from moving as if air currents cause it to ripple, these ripples creating pressure on the microphone ribbon, cause it to move and generate undesired currents.

The rivets 23 shown in Figure 1 may be used to hold the lining and wire gauze in place wi the shield;

In certain cases I have found that a single screen is inadequate to prevent air'currents from affecting the ribbon. In such cases, I have successfully used a double screen of the type shown in Figure 5. The inner screen may be made of the same materials as that shown in Figure 4 and curved as shown to surround the microphone ribbon and pole pieces. The outer perforated section 19, silk lining 21 and wire gauze 22 may be secured to the magnet structure by the screws 24 as shown or by any other desired means. A second or outer screen 25, preferably made of perforated metal in the same manner as the screen 19 of Figure 1 surrounds the entire microphone with its inner screen. This structure provides a mass of somewhat confined air between the outer and inner protective screens which I find has substantially no effect on the frequency characteristic of an amplifier and at the same time is very useful in locations where there are considerable air or wind currents which would otherwise affect the proper action of the transmitter.

Figures 6 and 7 show on a reduced scale a modified form of an outer screen which is adapted for use with the arrangement of Figure 1. This screen is in general cylindrical and consists of the outer perforated casing 19. silk lining 21 and inner wire gauze 22. As shown, the screen is split slightly near its top as shown at I ,26 and has clamping portions 27 near its top provided with-bolt and lock nut 28. The inside diameter of this screen is such that it may be slid over the lower cylindrical portion 1 of the main framework of Figure 1 and clamped in position by lock nut 28. When so used the transmitter of Figure 1 is protected by the inner screen 19 and the outer cylindrical screen described and by the mass \of somewhat confined air between'said screens.

As described in my application, Serial No. 528,598, the microphone ribbon is essentially a thin strip of flexible metal which may be crimped as shown in Figure 2 if desired. It has been found that an air gap substantially 7/32 of an inch in width (distance between pole pieces 16) and a ribbon'just slightly smaller gives good results, there only being a small clearance be-,- tween the edges of it and the pole pieces. 1..

It has been found that a microphone of the type described is highly directional. If a sound wave comes from a source at right angles to the plane of the ribbon, i. e., the plane of the apparatus itself, the sound wave has a maximum efiect'on the ribbon. But when the sound wave is from a source in the plane of the ribbon it has no effect on it as the instantaneous sound wave "pres sures on opposite sides thereof are equal. When the source of sound is at an intermediate point,' i. e., a point between the plane of the ribbon'and the perpendicular to the ribbon, its eflect is equal to the effect when the source is perpendicular to the ribbon, times the cosine of the angle between the perpendicular and the line from the source to the ribbon.

Use may be made of this directional property as illustrated in Figure 3. Here a' clamp 29 is shown which engages the cable'30 running to the five prong plug 6 at the top of the transmitter unit and supports the entire unit for pivotal movement about a vertical axis. A cord or other flexible member 31 has its ends secured to the lugs 9 at the top of the cover 7 and its other end looped about a pulley 32 which is pivotally mounted on a spindle 33 and having a handle 34. In practice pulley 32 may be located from 3 to 20 feet or more from the microphone. The moving picture camera 35 is preferably mounted so that it may view the scene of action and at the same time be in the plane of the microphone ribbon. Noise generated by the moving camera parts is then not picked up by the microphone. As the actor or sound source S moves about on the scene of action the sound monitor by slightly rotating pulley 32 may cause the microphone unit to move so that the plane of its ribbon is kept substantially normal to the line OS between the sound source and the microphone. In this way the microphone is kept operating at high efliciency during the motion of the sound source. This arrangement is found to provide a convenient means for controlling the position of the microphone unit from a remote point, such as the camera location, as pulley 32 may be located adjacent the camera so that its operator mayalso direct the microphone, if desired.

The transformer to which the ends of the microphone ribbon are coupled conductively, the output transformer of the amplifier and the component parts of the three stage resistance coupled amplifier are all mounted in the upper portion of frame 1 and are all protected and shielded by metal cover 7 when it is in place.

In the foregoing description and drawings several approved modifications of the invention have been disclosed and described. It is understood, however, that it is not intended to limit the invention to the modifications shown, as various other arrangements can be made without departing from the spirit of the invention. The invention is intended to be limited only by the scope of the appended claims.

I claim:

1. In combination with a microphone, a shield for said microphone comprising a casing having apertures therein, a closely woven fabric contacting the surface of said casing and a second casing spaced apart from said first named casing and enclosing it and the microphone, said second casing also provided with a plurality of apertures therein to transmit sound waves.

2. A microphone transmitter including a diaphragm, a frame supporting said transmitter, said frame having a curved cylindrical end portion above said transmitter, and a shield engaging said end portion and enclosing said transmitter, said shield having a plurality of apertures therein whereby sound waves incident thereon may be eificiently communicated to the opposite sides of said diaphragm, the end portion of said shield being provided with clamping means for securing it rigidly against said end portion.

3. In combination with a microphone, a shield for said microphone including a perforated casing, a closely woven fabric contacting with the surface of said casing, and a second perforated casing spaced from and enclosing said first casing and said microphone, the joint area and arrangement of said perforations being such as to produce no substantial attenuation to the transmission of audio frequency waves of the order of 5,000 cycles per second.

4. In combination, a microphone transmitter including a ribbon diaphragm, a shield for said transmitter comprising a perforated sheet metal casing provided with apertures having a joint area of approximately one-fourth of the total area of the shield and the said apertures being of uniform size and spacing whereby substantially no attenuation of the highest audio frequencies to be transmitted is produced.

5. In combination, a microphone transmitter including a ribbon diaphragm, a shield for said transmitter comprising a perforated sheet metal casing provided with apertures having a joint area of approximately one-fourth of the total area of the shield and the said apertures being of uniform size and spacing whereby substantially no attenuation of the highest audio frequencies to be transmitted is produced, and a fabric lining for said shield composed of threads so closely spaced as to prevent wind or air currents striking said perforations from striking or moving said microphone ribbon.

HARRY F. OLSON. 

